Polymers having reactive silyl groups or compositions comprising such polymers can be hydrolyzed and condensed in the presence of water and organometal catalysts. Suitable known catalysts for curable compositions include organometallic compounds employing metals such as Sn, Ti, Zn or Ca. Organotin compounds such as, for example, dibutyltin dilaurate (DBTDL) are widely used as condensation cure catalysts to accelerate the moisture assisted curing of a number of different polyorganosiloxanes and non-silicone polymers having reactive terminal silyl groups such as room temperature vulcanizing (RTV) formulations including RTV-1 and RTV-2 formulations. Environmental regulatory agencies and directives, however, have increased or are expected to increase restrictions on the use of organotin compounds in formulated products. For example, while formulations with greater than 0.5 wt. % dibutyltin presently require labeling as toxic with reproductive 1B classification, dibutyltin-containing formulations are proposed to be completely phased out in consumer applications during the next 4-6 years.
Alternative organotin compounds such as dioctyltin compounds and dimethyltin compounds can only be considered as a short-term remedial plan, as these organotin compounds may also be regulated in the future. As alternatives to tin catalysts, efforts have been made to identify non-Sn metal-based catalysts that accelerate the condensation curing of moisture curable silicones and non-silicones. Desirably, substitutes for organotin catalysts should exhibit properties similar to organotin compounds in terms of curing, storage, and appearance. Non-tin catalysts would also desirably initiate the condensation reaction of the selected polymers and complete this reaction upon the surface and may be in the bulk in a desired time schedule. There are therefore many proposals for the replacement of organometallic tin compounds by other organometallic compounds. These compounds comprise metals such as Ca, Ce, Bi, Fe, Mo, Mn, Pb, Ti, V, Zn and Y. All of these metals have specific advantages and disadvantages in view of replacing tin compounds perfectly. Therefore, there is still a need to overcome some of the weaknesses of possible metal compounds as suitable catalyst for condensation cure reaction including the behavior of uncured and cured compositions to maintain the ability to adhere onto the surface of several substrates. Another problem necessary to be solved in the replacement of organo-tin compounds is for the reactive composition to maintain its ability to cure (when exposed to humidity or ambient air) after storage in a sealed cartridge.
The cure chemistry of these moisture-curable compositions can vary based upon the nature of the polymers and their moisture-curable groups. For example, alkoxysilyl groups first hydrolyze to give silanol functionalities, which then condense with the extrusion of water to give the siloxane network. Such compositions typically comprise an alkoxysilyl- or silanol-functional polymer and a crosslinking agent. Tri- and tetraalkoxysilanes are commonly used as crosslinking agents and will react with water or directly with silanol groups to crosslink the system. However, for compositions comprising hydridosilyl groups or both hydridosilyl and silanol functionalities, such a crosslinking agent is not required. In fact, due to the multitude of hydridosilyl groups present, the hydridosilyl-containing compound is often referred to as the crosslinking agent. In these compositions, hydridosilyl groups may react with water to give silanol functionalities or they may react directly with silanol groups to form siloxane bonds with extrusion of hydrogen gas. For transition-metal-catalyzed compositions comprising a hydridosilyl-containing compound, inhibitors are commonly used to ensure adequate shelf life or pot life.